1 or 2 lights?
Well, both is nice, but let’s examine the downside of both bulbs.
If one light keeps burning out, and not the other, then there is another problem, a short, or a cracked casing.
Also, the extremely hot, bright halogen bulbs have a warning, don’t get grease on them. Even the oils in your fingers can make them fail. Buy a pack of vinyl gloves to do the replacements with.
Remember bulbs are rated in hours, but that varies by the number of times they go on / go off (thermal cycles). If you drive longer trips, they tend to last more hours, if you tend to make alot of short trips, they will burn out quicker.
I’m cheap. When one goes out, I but two bulbs, replace the burned out one and I will have #2 ready.
The last time I had a headlamp go out, I was driving back from NY to Ohio and as it got dark I found I no longer had head lights. I was happy to have fog lights.